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New study unveils ultrathin boron nitride films for next-generation electronics

Posted By Graphene Council, Friday, June 26, 2020
An international team of researchers, affiliated with UNIST has unveiled a novel material that could enable major leaps in the miniaturization of electronic devices. Published in the prestigious journal Nature, this study represent a significant achievement for future electronics.

This breakthrough comes from a research, conducted by Professor Hyeon Suk Shin (School of Natual Sciences, UNIST) and Principal Researcher Dr. Hyeon-Jin Shin from Samsung Advanced Institute of Technology (SAIT), in collaboration with Graphene Flagship researchers from University of Cambridge (UK) and Catalan Institute of Nanoscience and Nanotechnology (ICN2, Spain).

In this study, the team successfully demonstrated the synthesis of thin film of amorphous boron nitride (a-BN) with extremely low dielectric constant as well as high breakdown voltage and superior metal barrier properties. The research team noted that this newly fabricated material has great potential as interconnect insulators in the next-generation of electronic circuits.

In the ongoing process of miniaturization of logic and memory devices in electronic circuits, minimizing the dimensions of interconencts - metal wires that link the different device components on the chip - is crucial to guarantee improved performance and faster response of the device. Extensive research efforts have been devoted to decreasing the resistance of scaled interconnects because integration of dielectrics using complementary metal oxide semiconductor (CMOS) compatible processes has proven to be exceptionally challenging. According to the research team, the required interconnect isolation materials should not only possess low relative dielectric constants (referred to as k-values), but should also be thermally, chemically, and mechanically stable.

There has been an ongoing quest to obtain materials with ultra-low-k (relative permittivity around or below 2) avoiding the artificial addition of pores in the thin film in the semiconductor industry for at least the past 20 years. Several attempts had been made to develop materials with desired characteristics, yet those materials have failed to be successfully integrated in interconnects due to poor mechanical properties or poor chemical stability upon integration, causing reliability failures.

In this study, the joint research has succeeded in demonstrating a Back-End-ofthe-Line (BEOL) compatible approach to grow amorphous boron nitride (a-BN) with extremely low-k dielectrics. In particular, they synthesized approximately 3 nm thin a-BN on a Si substrate, using low temperature remote inductively coupled plasma-chemical vapour deposition (ICP-CVD). The resulting material showed an extremely low dielectric constant in the range of 1.78, which is 30% lower than the dielectric constant of currently available insulators.

In this study, the joint research has succeeded in demonstrating a Back-End-ofthe-Line (BEOL) compatible approach to grow amorphous boron nitride (a-BN) with extremely low-k dielectrics. In particular, they synthesized approximately 3 nm thin a-BN on a Si substrate, using low temperature remote inductively coupled plasma-chemical vapour deposition (ICP-CVD). The resulting material showed an extremely low dielectric constant in the range of 1.78, which is 30% lower than the dielectric constant of currently available insulators.

"We found that temperature was the most important parameter with ideal a-BN film deposition occurring at 400° C," says Seokmo Hong in the Doctoral program of Natural Sciences, the first author of the study. "This material with ultra-low-k also manifests a high breakdown voltage and likely superior metal barrier properties, making the film very attractive for practical electronic applications."

Angle-dependent near-edge X-ray absorption fine structure (NEXAFS) measured in partial electron-yield (PEY) mode at Pohang Light Source-II 4D beam line was also used to investigate the chemical and electronic structures of a-BN. Their findings indicated that the irregular, random atomic arrangement causes the dielectric constant value to drop.

The new material also manifests excellent mechanical properties of high strength. Moreover, when researchers tested the diffusion barrier properties of a-BN in very harsh conditions, they found it can prevent metal atom migration from the interconnects into the insulator. This result will help resolves a long-standing issue of interconnects in CMOS integrated circuit fabrication, enabling further miniaturaization of electronic devices.

"Development of electrically, mechanically and thermally robust low-k materials (k < 2) has long been technically challenging," says Dr. Hyeon-Jin Shin from Samsung Advanced Institute of Technology (SAIT). "Our research is also a great example that shows companies and academic institutions working together to create greater synergy."

"Our results demonstrate that the amorphous counterpart of two-dimensional hexagonal BN possesses the ideal low-k dielectric characteristics for high-performance electronics," says Professor Shin. "If they are commercialized, it will be a great help in overcoming the crisis looming over the semiconductor industry."

Tags:  boron nitride  Electronics  Graphene  hexagonal boron nitride  Hyeon Suk Shin  Hyeon-Jin Shin  Samsung Advanced Institute of Technology  UNIST 

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The Physics behind Twisted Bilayer Graphene

Posted By Graphene Council, Tuesday, May 26, 2020
An international team of researchers, including ICFO Prof. Dmitri Efetov, give a thorough review in Nature Physics on the status and prospects of the physics behind stacked monolayers of 2D materials.

Strongly correlated quantum materials are excellent testbeds for complex quantum phases of matter since they have shown to give rise to spectacular phenomenology, from high-temperature superconductivity to the emergence of states with long-range quantum entanglement. However, their complexity and the vast amount of system variables have so far hindered scientists to obtain a more complete, thorough understanding of its microscopic mechanisms.

Van der Waals heterostructures consist of individual layers of two-dimensional atomic materials such as graphene, hexagonal boron nitride, and transition metal chalcogenides, which are vertically stacked on top of each other. As the relative angle between the crystals can be freely chosen, this creates a new capability in physics – a concept which is called Twistronics. Scientists have recently found that by overlaying two individual layers of these 2D materials, complex heterostructures moiré structures can be created, which host an amazing realm of unexplored and undiscovered physical phenomena.

Recently and considered one of major scientific achievements in these last two years, researchers found that, when tuning one of these systems, specifically in a twisted bi-layer graphene system, it was possible to drive the system from exhibiting strongly correlated states to presenting clear superconductivity features. That is, by changing the electrical charge carrier density within the device with a nearby capacitor, the material could be tuned from behaving as an insulator, to behaving as a superconductor, or even an exotic orbital magnet with non-trivial topological texture – a phase never observed before. Sor far, there is still no theoretical approach that can precisely explain such complex and exceptionally rich physics, and in particular, how these all these states may be linked or connected with each other and why they occur in such order.

In a recent study published in Nature Physics, researchers Leon Balents, Cory R. Dean, ICFO Prof. Dmitri K. Efetov and Andrea F. Young give a thorough report on the status and the prospects of these systems and the physics that arises from them. They focus on understanding the patterns that are created when two individual monolayers of these materials, called moiré patterns, are stacked in a specific way, discuss the engineering in van der Waals heterostructures as well as explore how different phenomena emerge from the moiré flat bands that are formed.

Flat bands are advantageous because they guarantee a large density of states, which amplifies the effects of interactions. Bearing this in mind, the researchers have focused their study on moiré systems in Twisted Bi Layer Graphene (tBLG) in particular. They have studied various systems in which a small mismatch in periodicity of the pattern, introduced either by lattice mismatch or rotational misalignment, results in different physical scenarios, may it be correlating states, insulating states, superconductivity, etc. From these, they search, among other things, to understand and find answers to what may be the nature of the insulating states, what is the origin and nature of the observed superconductivity? How strong are analogies to other correlated electronic systems, such as high-temperature superconducting cuprates? This study is a step forward in understanding and setting the basis for the theory that may be capable of explaining in a future all the rich and very complex physics behind these novel materials and systems. Being able to control and manipulate such systems, and really understanding what is occurring inside them, will be a major advancement, if not a revolutionary shift, in the engineering of these materials and the development of applications for future innovative and disruptive technologies.

Tags:  2D Materials  Andrea F. Young  Cory R. Dean  Dmitri K. Efetov  Graphene  Hexagonal boron nitride  Leon Balents  quantum materials 

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Oriented hexagonal boron nitride foster new type of information carrier

Posted By Graphene Council, Tuesday, May 26, 2020
Valleytronics gives rise to valley current, a stable, dissipationless current which is driven by a pseudo-magnetic field, Berry curvature. This gives rise to valletronics based information processing and storage technology. A pre-requisite for the emergence of Berry curvature is either a broken inversion symmetry or a broken time-reversal symmetry. Thus two-dimensional materials such as transition metal dichalcogenides and gated bilayer graphene are widely studied for valleytronics as they exhibit broken inversion symmetry.

For most of the studies related to graphene and other two-dimensional materials, these materials are encapsulated with hexagonal boron nitride (hBN), a wide band gap material which has comparable lattice parameter to that of graphene. Encapsulation with hBN layer protects the graphene and other two-dimensional materials from unwanted adsorption of stray molecules while keeping their properties intact. hBN also acts as a smooth twodimensional substrate unlike SiO2 which is highly non-uniform, increasing the mobility of carriers in graphene. However, most of the valleytronics studies on bilayer graphene with hBN encapsulation has not taken into account the effect of hBN layer in breaking the layer symmetry of bilayer graphene and inducing Berry curvature.

This is why Japan Advanced Institute of Science and Technology (JAIST) postdoc Afsal Kareekunnan, senior lecturer Manoharan Muruganathan and Professor Hiroshi Mizuta decided it was vital to take into account the effect of hBN as a substrate and as an encapsulation layer on the valleytronics properties of bilayer graphene. By using first-principles calculations, they have found that for hBN/bilayer graphene commensurate heterostructures, the configuration, as well as the orientation of the hBN layer, has an immense effect on the polarity as well as the magnitude of the Berry curvature.

For non-encapsulated hBN/bilayer graphene heterostructure, where hBN is present only at the bottom, the layer symmetry is broken due to the difference in the potential experienced by the two layers of the bilayer graphene. This layer asymmetry induces a non-zero Berry curvature. However, encapsulation of the bilayer graphene with hBN (where the top and bottom hBN are out of phase with each other) nullifies the effect of hBN and drives the system towards symmetry, reducing the magnitude of the Berry curvature. A small Berry curvature which is still present is the feature of pristine bilayer graphene where the spontaneous charge transfer from the valleys to one of the layers results in a slight asymmetry between the layers as reported by the group earlier. Nonetheless, encapsulating bilayer graphene with the top and bottom hBN in phase with each other enhances the effect of hBN, leading to an increase in the asymmetry between the layers and a large Berry curvature. This is due to the asymmetric potential experienced by the two layers of bilayer graphene from the top and bottom hBN. The group has also found that the magnitude and the polarity of the Berry curvature can be tuned in all the above-mentioned cases with the application of an out-of-plane electric field.

"We believe that, from both theoretical and experimental perspective, such precise analysis of the effect of the use of hBN both as a substrate and as an encapsulation layer for graphene-based devices gives deep insight into the system which has great potential to be an ideal valleytronic material," Professor Mizuta said.

Tags:  Afsal Kareekunnan  Graphene  Hexagonal boron nitride  Hiroshi Mizuta  Japan Advanced Institute of Science and Technology  Manoharan Muruganathan 

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High-quality boron nitride grown at atmospheric pressure

Posted By Graphene Council, Wednesday, April 22, 2020
Graphene Flagship researchers at RWTH Aachen University, Germany and ONERA-CNRS, France, in collaboration with researchers at the Peter Grunberg Institute, Germany, the University of Versailles, France, and Kansas State University, US, have reported a significant step forward in growing monoisotopic hexagonal boron nitride at atmospheric pressure for the production of large and very high-quality crystals.

Hexagonal boron nitride (hBN) is the unsung hero of graphene-based devices. Much progress over the last decade was enabled by the realisation that 'sandwiching' graphene between two hBN crystals can significantly improve the quality and performance of the resulting devices. This finding paved the way to a series of exciting developments, including the discoveries of exotic effects such as magic-angle superconductivity and proof-of-concept demonstrations of sensors with unrivalled sensitivity.

Until now, the most widely used hBN crystals came from the National Institute of Material Science in Tsukuba, Japan. These crystals are grown using a process at high temperatures (over 1500°C) and extremely high pressures (over 40,000 times atmospheric pressure). "The pioneering contribution by the Japanase researchers Taniguchi and Watanabe to graphene research is invaluable", begins Christoph Stampfer from Graphene Flagship Partner RWTH Aachen University, Germany. "They provide hundreds of labs around the world with ultra-pure hBN at no charge. Without their contribution, a lot of what we are doing today would not be possible."

However, this hBN growth method comes with some limitations. Among them is the small crystal size, which is limited to a few 100 µm, and the complexity of the growth process. This is suitable for fundamental research, but beyond this, a method with better scalability is needed. Now Graphene Flagship researchers tested hBN crystals grown with a new methodology that works at atmospheric pressure, developed by a team of researchers led by James Edgar at Kansas State University, US. This new approach shows great promise for more demanding research and production.

"I was very excited when Edgar proposed that we test the quality of his hBN", says Stampfer. "His growth method could be suitable for large-scale production". The method for growing hBN at atmospheric pressure is indeed much simpler and cheaper than previous alternatives and allows for the isotopic concentration to be controlled.

"The hBN crystals we received were the largest I have ever seen, and they were all based either on isotopically pure boron-10 or boron-11" says Jens Sonntag, a graduate student at Graphene Flagship Partner RWTH Aachen University. Sonntag tested the quality of the flakes first using confocal Raman spectroscopy. In addition, Graphene Flagship partners in ONERA-CNRS, France, led by Annick Loiseau, carried out advanced luminescence measurements. Both measurements indicated high isotope purity and high crystal quality.

However, the strongest evidence for the high hBN qualitycame from transport measurements performed on devices containing graphene sandwiched between monoisotopic hBN. They showed equivalent performance to a state-of-the-art device based on hBN from Japan, with better performance in some areas.

"This is a clear indication of the extremely high quality of these hBN crystals," says Stampfer. "This is great news for the whole graphene community, because it shows that it is, in principle, possible to produce high quality hBN on a large scale, bringing us one step closer to real applications based on high-performance graphene electronics and optoelectronics. Furthermore, the possibility of controlling the isotopic concentration of the crystals opens the door to experiments that were not possible before."

Mar García-Hernández, Work Package Leader for Enabling Materials, adds: "Free-standing graphene, being the thinnest material known, exhibits a large surface area and, therefore, is extremely sensitive to its surrounding environment, which, in turn, results in substantial degradation of its exceptional properties. However, there is a clear strategy to avoid these deleterious effects: encapsulating graphene between two protective layers."

García-Hernández continues: "When graphene is encapsulated by hBN, it reveals its intrinsic properties. This makes hBN an essential material to integrate graphene into current technologies and demonstrates the importance of devising new scalable synthetic routes for large-scale hBN production. This work not only provides a new and simpler path to produce high-quality hBN crystals on a large scale, but it also enables the production of monoisotopic material, which further reduces the degradation of graphene when encapsulated by two layers."

Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship and Chair of its Management Panel, adds: "This is a nice example of collaboration between the EU and the US, which we fostered via numerous bilateral workshops. Devising alternative approaches to produce high-quality hBN crystals is crucial to enable us to exploit the ultimate properties of graphene in opto-electronics applications. Furthermore, this work will lead to significant progress in fundamental research."

Tags:  Andrea C. Ferrari  Christoph Stampfer  Graphene  Graphene Flagship  Hexagonal boron nitride  Mar García-Hernández  ONERA-CNRS  optoelectronics  RWTH Aachen University  Sensors 

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Substances trapped in graphene nanobubbles exhibit unusual properties

Posted By Graphene Council, Wednesday, April 15, 2020
Skolkovo Institute of Science and Technology (Skoltech) scientists modeled the behavior of nanobubbles appearing in van der Waals heterostructures and the behavior of substances trapped inside the bubbles. In the future, the new model will help obtain equations of state for substances in nano-volumes, opening up new opportunities for the extraction of hydrocarbons from rock with large amounts of micro- and nanopores.

The results of the study were published in the Journal of Chemical Physics ("Model of graphene nanobubble: Combining classical density functional and elasticity theories").

The van der Waals nanostructures hold much promise for the study of tiniest samples with volumes from 1 cubic micron down to several cubic nanometers. These atomically thin layers of two-dimensional materials, such as graphene, hexagonal boron nitride (hBN) and dichalcogenides of transition metals, are held together by weak van der Waals interaction only.

Inserting a sample between the layers separates the upper and bottom layers, making the upper layer lift to form a nanobubble. The resulting structure will then become available for transmission electron and atomic force microscopy, providing an insight into ? the structure of the substance inside the bubble.

The properties exhibited by substances inside the van der Waals nanobubbles are quite unusual. For example, water trapped inside a nanobubble displays a tenfold decrease in its dielectric constant and etches the diamond surface (Nature Communications, "A hydrothermal anvil made of graphene nanobubbles on diamond"), something it would never do under normal conditions. Argon which typically exists in liquid form when in large quantities can become solid at the same pressure if trapped inside very small nanobubbles with a radius of less than 50 nanometers.

Scientists led by professor Iskander Akhatov of the Skoltech Center for Design, Manufacturing and Materials (CDMM) built a universal numerical model of a nanobubble that helps predict the bubble’s shape under certain thermodynamic conditions and describe the molecular structure of the substance trapped inside.

“In a practical sense, the bubbles in the van der Waals structures are most often regarded as flaws that experimenters are eager to get rid of. However, from the standpoint of straintronics, the bubbles create strain, and its effect on the electronic structure can be used to create practical devices, such as transistors, logic elements and ROM,” Petr Zhilyaev, a senior research scientist at Skoltech, commented on the study.

“In our recent study, we created a model which describes a specific shape that flat nanobubbles assume in the subnanometer dimension range only. We discovered that the vertical size of these nanostructures can only take discrete values divisible by the size of the molecules trapped. In addition, the model enables changing the size of nanobubbles by controlling the temperature of the system and the physicochemical parameters of the materials,” explained a senior research scientist at Skoltech, Timur Aslyamov.

Tags:  Graphene  hexagonal boron nitride  Iskander Akhatov  Petr Zhilyaev  Skolkovo Institute of Science and Technology  Timur Aslyamov 

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An ultimate one-dimensional electronic channel in hexagonal boron nitride

Posted By Graphene Council, Wednesday, March 11, 2020
In the field of 2D electronics, the norm used to be that graphene is the main protagonist and hexagonal boron nitride (hBN) is its insulating passive support. Researchers of the Center for Multidimensional Carbon Materials (CMCM) within the Institute for Basic Science (IBS, South Korea) made a discovery that might change the role of hBN. They have reported that stacking of ultrathin sheets of hBN in a particular way creates a conducting boundary with zero bandgap. In other words, the same material could block the flow of electrons, as a good insulator, and also conduct electricity in a specific location. Published in the journal Science Advances, this result is expected to raise interest in hBN by giving it a more active part in 2D electronics.

Similarly to graphene, hBN is a 2D material with high chemical, mechanical and thermal stability. hBN sheets resemble a chicken wire, and are made of hexagonal rings of alternating boron and nitrogen atoms, strongly bound together. However, unlike graphene, hBN is an insulator with a large bandgap of more than five electronVolts, which limits its applications.

“In contrast to the wide spectrum of proposed applications for graphene, hexagonal boron nitride is often regarded as an inert material, largely confined as substrate or electron barrier for 2D material-based devices. When we began this research, we were convinced that reducing the bandgap of hBN could give to this material the versatility of graphene,” says the first author, PARK Hyo Ju.

Several attempts to lower the bandgap of hBN have been mostly ineffective because of its strong covalent boron-nitrogen bonds and chemical inertness. IBS researchers in collaboration with colleagues of Ulsan National Institute of Science and Technology (UNIST), Sejong University, Korea, and Nanyang Technological University, Singapore, managed to produce a particular stacking boundary of a few hBN layers having a bandgap of zero electronVolts.

Depending on how the hBN sheets are piled up, the material can assume different configurations. For example, in the so-called AA′ arrangement, the atoms in one layer are aligned directly on the top of atoms in another layer, but successive layers are rotated such that boron is located on nitrogen and nitrogen on boron atoms. In another type of layout, known as AB, half of the atoms of one layer lie directly over the center of the hexagonal rings of the lower sheet, and the other atoms overlap with the atoms underneath.

For the first time, the team has reported atomically sharp AA′/AB stacking boundaries formed in few-layer hBN grown by chemical vapor deposition. Characterized by a line of oblong hexagonal rings, this specific boundary has zero bandgap. To confirm this result, the research performed several simulations and tests via transmission electron microscopy, density functional theory calculations, and ab initio molecular dynamics simulations.

“An atomic conducting channel expands the application range of boron nitride infinitely, and opens new possibilities for all-hBN or all 2D nanoelectronic devices,” points out the corresponding author LEE Zonghoon.

Tags:  2D materials  Graphene  hexagonal boron nitride  Institute for Basic Science  PARK Hyo Ju 

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Study puts spin into quantum technologies

Posted By Graphene Council, Thursday, February 27, 2020

A team of international scientists investigating how to control the spin of atom-like impurities in 2D materials have observed the dependence of the atom's energy on an external magnetic field for the first time.

The results of the study, published in Nature Materials, will be of interest to both academic and industry research groups working on the development of future quantum applications, the researchers say.


Researchers led by Prof Vladimir Dyakonov at the University of Würzburg in collaboration with scientists from the University of Technology Sydney (UTS), the Kazan Federal University and the Universidade Federal de Minas Gerais, demonstrated the ability to control the spin of atom-like impurities in 2D material hexagonal boron-nitride. By combining laser and microwave excitation the researchers were able to change the spin states, for example "up" to "down", of atom-like impurities hosted in the material and show the dependence of their energy on an external magnetic field.

This is the first time that the phenomenon has been observed in a material that is made of a single sheet of atoms like graphene. The researchers say that this newly demonstrated quantum spin-optical properties, combined with the ease of integrating with other 2D materials and devices, establishes hexagonal boron-nitride as an intriguing candidate for advanced quantum technology hardware.

"2D atomic crystals are currently some of the most studied materials in condensed matter physics and materials science," says UTS physicist Dr Mehran Kianinia, a co-author of the study.

"Their physics is intriguing from a fundamental point of view, but beyond that, we can think of stacking different 2D crystals to create completely new materials, heterostructures and devices with specific designer properties," he says.

UTS researcher, Dr Carlo Bradac, a senior co-author of the study says that in addition to adding another unique property, to an already impressive range of properties for a 2D material, the discovery has enormous potential for the field of quantum sensing.

"What really excites me is the potential [in the context of quantum sensing]. These spins are sensitive to their immediate surroundings. Unlike 3D solids, where the atom-like system can be as far as a few nanometres from the object to sense, here the controllable spin is right at the surface. Our hope is to use these individual spins as tiny sensors and map, with unprecedented spatial resolution, variations in temperature, as well as magnetic and electric fields onto variations in spin" Dr Bradac says.

"Imagine, for instance, being able to measure minuscule magnetic fields with sensors as small as single atoms. The possibilities are far reaching and range from nuclear magnetic resonance spectroscopy for nanoscale medical diagnostic and material chemistry to GPS-free navigation using the Earth's magnetic field," he says.

However quantum-based nanoscale magnetometry is "just one area where controlling single spins in solids is useful" says senior author of the study UTS Professor Igor Aharonovich.

"Beyond quantum sensing, many quantum computing and quantum communication applications rely on our ability to control the spin-state--zero, one and anything in between--of single atom-like systems in solid host materials. This allows us to encode, store and transfer information in the form of quantum bits or qubits," he says.

Amongst many others, this research highlights how scientists are quickly becoming masters in the craft of manipulating objects in the quantum regime. In fact, achievements like Lockheed Martin's Black Ice project and Google's quantum supremacy are proof that we are striding away from mere proof-of-concept experiments towards real world, quantum-enabled solutions to practical problems.

Tags:  2D materials  Graphene  Hexagonal boron nitride  Kazan Federal University  Nature Materials  Universidade Federal de Minas Gerais  University of Technology Sydney  University of Wurzburg  Vladimir Dyakonov 

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How tiny misalignments in encapsulated graphene lead to a strong modification of its electronic properties

Posted By Graphene Council, Friday, January 31, 2020
Researchers at the University of Antwerp explain how higher order supermoiré periodic modulations due to the encapsulation of graphene between hexagonal boron nitride affect the electronic and structural properties of graphene, as revealed in three recent independent experiments.

High quality graphene samples are of high importance for obtaining and exploiting its theoretically described properties. Utilizing an adequate substrate reduces the corrugation and improves otherwise disorder limited properties of graphene.

Hexagonal boron nitride (hBN) is a particularly good choice, since it preserves perfectly the graphene structure, while providing a flat insulating surface. Still, this applies only if the two monolayers are misaligned. Otherwise, the van der Waals interaction induces structural relaxation on the scale of the moiré pattern formed between the two layers and modifies the electronic properties due to the periodic moiré perturbation.

Similar arguments apply if graphene is encapsulated and closely aligned to two hBN layers. In this case the effect is enhanced since both layers are expected to contribute. Furthermore, close alignment, on the order of 0.5 degrees, between the layers is responsible for the appearance of a new form of periodic supermoiré modulation, which alters graphene on a larger spatial scale, but smaller energy scale.

Recent experimental observation of such effects are a consequence of significant improvements in the experimental manipulation techniques, and among others, the possibility to rotate individual layers with high precision (Wang et al. 2019a; Wang et al. 2019b; Finney et al. 2019 – see references at the end of this article).

In their recent paper published in Nano Letters ("Double moiré with a twist: supermoiré in encapsulated graphene"), Anđelković et al. reveal under which condition the supermoiré effect appears, and how it alters the structural and electronic properties of graphene.

They show, starting from a rigid hBN/graphene/hBN heterostructure, how the supermoiré appears as a simple geometrical consideration. Furthermore, they prove that relaxation effects in the three layers are expected to enhance the effects on the electronic band structure. The supermoiré induced modifications are significant: new low energy flat sub-bands and Dirac points appear, with strong effect on electronic transport properties. In most configurations the Dirac points are gapped, while flat bands are expected to enhance electron-electron correlations.

"These new twisting degrees of freedom in heterostructures are opening up new fundamental research directions in graphene, where strong electronic correlations are expected to complement the already superlative properties of graphene," said Dr. Lucian Covaci.

"The set of multi-scale numerical simulations developed by the University of Antwerp team allows for more realistic models, which will in turn allow for a more direct comparison with experimental observations," said Dr. Miša Anđelković, a co-developer of Pybinding, the tight-binding open source software that made the simulations possible.

With a new light shed on the understanding of more complex and interfering behaviour of van der Waals heterostructures it is possible to finely tune graphene’s electronic properties and reach regimes where twist induced phenomena, such as flat bands or the appearance of mini-gaps, reveal themselves more clearly.
 

Tags:  Graphene  hexagonal boron nitride  Lucian Covaci  Miša Anđelković  University of Antwerp 

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Researcher’s break the geometric limitations of moiré pattern in graphene heterostructures

Posted By Graphene Council, Wednesday, January 1, 2020
Researchers at The University of Manchester have uncovered interesting phenomena when multiple two-dimensional materials are combined into van der Waals heterostructures (layered ‘sandwiches’ of different materials).

These heterostructures are sometimes compared to Lego bricks – where the individual blocks represent different atomically thin crystals, such as graphene, and stacked on top of each other to form new devices.

Published in Science Advances, the team focus on how the different crystals begin to alter one another’s fundamental properties when brought into such close proximity. Of particular interest is when two crystals closely match and a moiré pattern forms. This moiré pattern has been shown to affect a range of properties in an increasing list of 2D materials. However, typically the geometry of the moiré pattern places a restriction on the nature and size of the effect.

A moiré pattern is due to the mismatch and rotation between the layers of each materials which produces a geometric pattern similar to a kaleidoscope.

Our results push through the geometric limitation for these systems and therefore present new opportunities to see more of such science, as well as new avenues for applications.
Zihao Wang and Colin Woods, School of Natural Science

The team have broken this restriction by combining moiré patterns into composite ‘super-moiré’ in graphene both aligning to substrate and encapsulation hexagonal boron nitride. The researchers demonstrate the nature of these composite super-moiré lattices by showing band structure modifications in graphene in the low-energy regime. Furthermore, they suggest that the results could provide new directions for research and devices fabrication.

Zihao Wang and Colin Woods authors of the paper said: “In recent years moiré pattern have allowed the observation of many exciting physical phenomena, from new long-lived excitonic states, Hofstadter’s Butterfly, and superconductivity.

Our results push through the geometric limitation for these systems and therefore present new opportunities to see more of such science, as well as new avenues for applications.”

Tags:  2D materials  Colin Woods  Graphene  hexagonal boron nitride  University of Manchester  Zihao Wang 

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Saving Moore’s Law

Posted By Graphene Council, Tuesday, December 31, 2019
It’s a well-known observation: The number of transistors on a microchip will double roughly every two years. And, thanks to advances in miniaturization and performance, this axiom, known as Moore’s Law, has held true since 1965, when Intel co-founder Gordon Moore first made that statement based on emerging trends in chip manufacturing at Intel. 

However, integrated circuits are hitting hard physical limits that are rendering Moore’s Law obsolete — elements on a dense integrated circuit (IC) can get only so small and so tightly packed together before they begin to interfere with each other and otherwise lose their functionality.

“Apart from fundamental physical limits to the scaling of transistor feature sizes below a few nanometers, there are significant challenges in terms of reducing power dissipation, as well as justifying the incurred cost of IC fabrication,” said Kaustav Banerjee, a professor of electrical and computer engineering at UC Santa Barbara. As a result, the very devices that we rely on for their steadily improving performance and versatility — computers, smartphones, internet-enabled gadgets — would also hit a limit, he said.

But according to Banerjee, one of world’s leading scientific minds in the field of nanoelectronics, there is a way to maintain Moore’s Law indefinitely, by taking advantage of relatively new and promising two-dimensional (2D) materials and combining them with monolithic 3D (M3D) integration practices to create ultra-compact, yet high-performing electronic chips that could overcome the challenges that face conventional integrated circuits. While Banerjee first disclosed this idea in a visionary article back in 2014, more detailed research evaluating this technology from his Nanoelectronics Research Lab was recently published in the IEEE Journal of the Electron Devices Society.

“Two-dimensional materials can be stable in their monolayer form with atomic scale thickness – 0.5 nanometer or 5 Angstroms for graphene (a conductor) and hexagonal-boron-nitride (an insulator), and ~6.5 Angstroms for 2D transition metal dichalcogenides (semiconductors) such as molybdenum-disulphide (MoS2) or tungsten-disulphide/diselenide (WS2/WSe2).” Banerjee said. “In addition, due to their layered nature, they offer pristine surfaces relatively free of defects and are excellent conductors of heat in the in-plane direction. All these properties, along with the possibility to directly synthesize these materials on top of prefabricated devices, offer unprecedented advantages over conventional 3D ICs that are already in the market or M3D integration with conventional electronic materials.”

The Benefits of Thinness 

According to the Banerjee Group’s study, there’s a limit to how thin conventional semiconductor materials can get before their desirable electronic properties begin to fade. 

“Thickness scaling of common semiconductor materials, such as Si, becomes challenging below a few nanometers due to rapid degradation of their mobility caused by the increase in electron scatterings from surface roughness,” Banerjee said. “In fact, below ~1 nm, conventional materials like Si or Ge may not be thermodynamically stable.”

On the other hand, atomically thin and stable 2D materials, such as graphene, hexagonal boron nitride (h-BN), and transitional metal dichalcogenides (MoS2, WS2, WSe2, etc) are highly space-efficient, thickness-wise. Moreover, due to their layered nature and pristine interfaces, the 2D semiconductors exhibit reasonably high mobilities and immunity against surface defects, according to the paper. In addition, 2D materials tend to be a lot more flexible than their conventional counterparts, which make them ideal for state-of-the-art electronics applications, such as flexible displays.  Stacked 2D materials, in contrast to their stacked 3D counterparts, meanwhile, can also minimize the inter-tier signal delays, thermal resistance, and reduce potential overheating.

By selecting certain 2D materials and stacking them, according to the researchers, not only does the monolithic 3D conserve precious space on the chip, but also allows for configuration based on the combined electronic properties of the materials.

For example, owing to the atomically-thin vertical dimensions of 2D materials, and carefully-designed inter-tier electrostatics with graphene shielding layer that also benefits from enhanced heat dissipation, aggressive scaling of tier thickness down to sub-μm can be achieved,” Banerjee said. “Such scaling allows over 10-folds higher integration density with respect to conventional 3D integration, and over 150% greater integration density with respect to conventional M3D integration, with plenty of room for further improvements.” 

“Thus, 2D materials can help realize the ultimate density scaling of integrated electronics — both laterally and vertically — which can usher an unprecedented era of innovation and economic growth for the worldwide semiconductor industry,” he added.

Manufacturing Outlook

As with many innovations with potential to become mainstream technologies, there are challenges to consider to pave the way toward their mass manufacturing. For monolithic 3D devices, the challenges are to be able to fabricate these components at relatively low temperatures (lower than 500 degrees Celsius) to avoid degradations and damages to prefabricated devices located in the lower tiers; electromagnetic interference; and heat dissipation.

Last year, Banerjee’s group demonstrated a CMOS compatible graphene synthesis method that essentially addressed the low-temperature and transfer-free synthesis challenge for graphene. Similar efforts are underway in his laboratory to synthesize other 2D materials directly on wafers at low temperatures.

“Additionally, careful design is needed to electrically shield the generated electromagnetic waves from affecting the operations of devices on adjacent or nearby tiers,” said Junkai Jiang, the lead author of the article and recent recipient of a doctoral degree in electrical and computer engineering from Banerjee’s laboratory. The researchers noted that by using a thin graphene shielding layer between tiers (preferably doped to enhance electromagnetic screening effect), interference can be prevented even as the vertical layers are scaled down. 

In terms of heat dissipation, the thinness of the material itself is conducive to allowing the heat from densely packed stacked components to dissipate efficiently. Kamyar Parto, a co-author of the study and a member of Banerjee’s lab, remarked that “the 2D materials have much higher in-plane thermal conductivity compared to thinned-down conventional materials like silicon, which helps fast lateral heat transport, thereby reducing the risks of any hot-spot formation.”  

“Ultimately, we envision heterogeneously integrated devices and technologies enabled by 2D materials to realize the world’s tallest and densest ‘chip-cities’ with unprecedented performance, storage capacity, and energy-efficiency,” he added.

Tags:  2D materials  Electronics  Graphene  Hexagonal boron nitride  Intel  Junkai Jiang  Kamyar Parto  Kaustav Banerjee  nanoelectronics  Semiconductor 

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